Friday, June 17, 2011

Lunar Eclipses: How do they work?

Yesterday the Sun, Earth and Moon were aligned exactly, with Earth in the middle. In effect sky gazers in much of the world witnessed a total lunar eclipse, said to be the longest in 21th century. Since the moon passed through near-center of the Earth's shadow, the eclipse lasted for a total of 1 hour and 40 minutes. It was visible over Europe and South America after sunset, over Africa and most of Asia, and Australia before sunrise. This was the first of the two lunar eclipses in 2011. Next one is due for December 10 and will be seen in most parts of the world, excluding South America and parts of Western Africa.

What is a lunar eclipse and when does it occur?

A lunar eclipse occurs when the Earth's shadow falls on the Moon. This can only happen when the Sun, Earth and Moon are aligned. In the process Earth obscures Sun's rays from reaching the Moon. Lunar eclipses can only occur at full moon - that is when the Moon is behind the Earth in relation to the Sun. But that raises an obvious question, why doesn't lunar eclipse occur every month?

Video explaining why lunar eclipse doesn't occur every month

As it turns out, the Moon's orbit is slightly inclined (by about 5 degrees) with respect to Earth's orbit around the Sun. As a result, the Moon crosses the Earth's orbital plane twice every month, at points called nodes. When the plane is crossed during Full Moon or New Moon (when nodes are aligned with the Sun and Earth), eclipses occur. The former causes a lunar eclipse, while the latter causes a solar eclipse.

Lunar orbit in relation to Earth's orbital plane at different times of the year

Why does the Moon turn red during lunar eclipse?

Lunar eclipse phases

During a total lunar eclipse, the Earth prevents Sun's rays from striking the Moon directly. Even though Moon is positioned behind the Earth, indirect sunlight can still reach and illuminate it. As the sunlight reaches Earth some of the rays are bent and redirected through the atmosphere and onto the Moon. In the process most of the blue light gets filtered out - ie. the more atmosphere in sunlight's way, the more blue and green rays get scattered. What remains is red sunlight because reddish wavelengths are more penetrating.